I don't believe that Mercedes sells individual seals for the steering box. To replace the seal(s) it's a good idea to remove the steering box and just go through it. After all, the other seals are just as old as the leaking one. You could try to match up the seal at a parts supply house or McMaster Carr if you could get it out in one piece. And you could replace the single seal with the box in the car. The manual boxes are much easier to reseal than the later P/S boxes. If it worked this long, doesn't it deserve a whole kit?
Regards,
Randy D.